This application claims priority to an application entitled xe2x80x9cPortable Communication Terminalxe2x80x9d, filed in the Japanese Patent Office on Nov. 13, 2000 and assigned Serial No. 2000-345452, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a portable communication terminal usable at multiple frequencies, and in particular, to a portable communication terminal, such as a portable telephone or a personal handyphone system (PHS), which is capable of achieving a reduction in specific absorption rate (SAR).
2. Description of the Related Art
As the use of portable communication terminals such as portable telephones has increased, much attention has been paid to their effects on the human body, in particular, the head, when it is exposed to electromagnetic waves radiated from those portable communication terminals. SAR is a measure of such effects of electromagnetic waves on the human body. In a portable communication terminal, current concentrates on the antenna of the terminal. As a result, electromagnetic fields are concentrated about the head of the user to which a radiation source, for example, the feeding point of the antenna, is closely positioned. This results in an increase in SAR. Currently, a number of research efforts are being made to achieve a reduction in SAR. Also, guidelines for the protection of the human body from electromagnetic waves are being established.
Monopole antennas have been typically used in general portable communication terminals. Such monopole antennas have an antenna structure including an antenna element and an antenna earth plate which resonate at a certain frequency. Antenna current flowing through the antenna causes earth current to flow through the PCB (Printed Circuit Board) serving as the earth plate.
It may be possible to achieve a reduction in SAR by reducing the intensity of near electromagnetic fields acting around the head of the user. However, it is difficult to control earth current. For this reason, it is difficult to control near electromagnetic fields in order to reduce the intensity thereof. As a result, it is difficult to reduce SAR in monopole type antenna systems.
In this regard, it is advantageous to use a dipole antenna (balanced power feeding antenna) involving no earth current flowing through the PCB.
Typically, feeding of power to such a dipole antenna is carried out by a coaxial cable (semirigid cable). Referring to FIG. 1a, the basic configuration of a dipole antenna is illustrated. In such a configuration, however, where feeding of power to a dipole antenna 10 is carried out by an unbalanced circuit, that is, a coaxial cable 11, leakage current is generated at the outer conductor of the coaxial cable 11. In order to prevent such a generation of leakage current, a balance/unbalance transformer, that is, balun, (not shown) is coupled between the dipole antenna 10 and the coaxial cable 11.
For such a balun, a transformer type balun shown in FIG. 2a is widely used. Of course, baluns of other types may be used. For example, a bridge type or a ladder type, which uses a lumped constant, may be used. Such types are illustrated in FIGS. 2b and 2c. 
However, portable communication terminals typically use a frequency band of about 1,000 MHz or more. Recently, systems using a higher frequency band have been developed. For a high frequency band, for example, microwaves of about 2,000 MHz or more adopted in wide-band code division multiple access (W-CDMA), the baluns of FIGS. 2a to 2c are impractical because an increased high frequency loss occurs.
Meanwhile, there is a balun called a xe2x80x9cbazookaxe2x80x9d. As shown in FIG. 1b, the balun, which is denoted by the reference numeral 12, is arranged around the tip of the coaxial cable 11 in the form of a cylinder while having a length of xc2xcxc2x7xcex. This balun is electrically open at the tip of the coaxial cable 11.
Such a bazooka type balun exhibits good electrical performance even in a high frequency band. However, where this bazooka type balun is applied to general portable communication terminals using a frequency of 800 MHz, the xc2xcxc2x7xcex length thereof corresponds to about 90 mm. Accordingly, the bazooka type balun is impractical.
Therefore, the present invention has been made in view of the above mentioned problems, and an object of the invention is to provide a bazooka type balun having a reduced size to be usable in a plurality of frequency bands.
In accordance with the present invention, this object is accomplished by providing a portable communication terminal comprising: an antenna printed circuit board mounted on a main printed circuit board provided with a transmitting/receiving circuit; a pair of antennas mounted on the antenna printed circuit board, the first one of the antennas being used at a first frequency with the second one of the antennas being used at a second frequency higher than the first frequency; a coaxial cable for connecting the first and second antennas to the transmitting/receiving circuit; and a cylindrical balance/unbalance transformer fitted around an antenna-side end of the coaxial cable and adapted to be usable at multiple frequencies, the balance/unbalance transformer including a first dielectric cylinder having a length corresponding to the first frequency, a second dielectric cylinder having a length corresponding to the second frequency, and a conductor cylinder, the second dielectric cylinder and the conductor cylinder being coaxially fitted in the first dielectric cylinder.